Demand for portable electronic devices is increasing each year. Example portable electronic devices include: laptop computers, personal data assistants (PDAs), cellular telephones, and electronic pagers. Portable electronic devices place high importance on total weight, size, and battery life for the devices. Many portable electronic devices employ rechargeable batteries such as Nickel-Cadmium (NiCad), Nickel-Metal-Hydride (NiMHi), Lithium-Ion (Li-Ion), and Lithium-Polymer based technologies.
In many portable power applications, a voltage that exceeds the battery voltage is required to operate certain circuits such as a video display. DC—DC converters are switching-type regulators that can be used to generate higher output voltages from a battery voltage. The output voltage is typically provided to a load circuit by varying the conduction time that is associated with a controlled device. Example controlled devices include transistors, gate-turn-on (GTO devices), thyristors, diodes, as well as others The frequency, duty cycle, and conduction time of the controlled device is varied to adjust the average output voltage to the load. Typical DC—DC converters are operated with some sort of oscillator circuit that provides a clock signal. The output voltage of the converter is also determined by the oscillation frequency associated with the clock signal.
For display applications such as stacked light emitting diodes (LEDs), the DC—DC converter often employs a constant frequency current mode control scheme. An example of a conventional closed loop control circuit (100) for driving LEDs is illustrated in FIG. 1. Circuit 100 includes an oscillator, an SR-type latch, an inductor (L1), two transistors (Q1, Q2), a Schottky diode (D1), two capacitors (C1, C2), three resistors (RSET, RSNS1, RSNS2), three amplifiers (A1 –A3), two driver circuits (DRV1, DRV2), a reference circuit (REF), a summer, and the LED stack (D2–D5).
At the start of each cycle of the oscillator, the SR latch is set and transistor Q1 is turned on via driver circuit DRV1. Amplifier A3 produces a sense voltage (VSNS1) by sensing the switching current from transistor Q1 via sense resistor RNSN1. The signal (VSUM) at the non-inverting input of the PWM comparator (A2) is determined by the switch current via VSNS1, summed together with a portion of the oscillation ramp signal. Amplifier A1 is an error amplifier that provides an error signal (VERR) by evaluating the drive current (ILED) via transistors Q2 and resistor RSNS2. The PWM comparator (A2) resets the SR latch and turns off transistor Q1 when the sum signal (VSUM) reaches the level set by the error signal (VERR). Thus, amplifier A1 and driver circuit DRV1 set the peak current level to keep the drive current (ILED) in regulation. Resistor RSET is adjusted to change the peak current level via a reference circuit (REF) and amplifier A1.